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.Dd October 15, 2025
.Dt CLOCK_GETTIME 2
.Os
.Sh NAME
.Nm clock_gettime ,
.Nm clock_settime ,
.Nm clock_getres
.Nd get/set/calibrate date and time
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Ft int
.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
.Ft int
.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
.Ft int
.Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
.Sh DESCRIPTION
The
.Fn clock_gettime
and
.Fn clock_settime
system calls allow the calling process to retrieve or set the value
used by a clock which is specified by
.Fa clock_id .
.Pp
The
.Fa clock_id
argument can be a value obtained from
.Xr clock_getcpuclockid 3
or
.Xr pthread_getcpuclockid 3
as well as the following values:
.Pp
.Bl -tag -width indent -compact
.It Dv CLOCK_REALTIME
.It Dv CLOCK_REALTIME_PRECISE
.It Dv CLOCK_REALTIME_FAST
.It Dv CLOCK_REALTIME_COARSE
Increments in SI seconds like a wall clock.
It uses a 1970 epoch and implements the UTC timescale.
The count of physical SI seconds since 1970, adjusted by subtracting the number
of positive leap seconds and adding the number of negative leap seconds.
Behavior during a leap second is not defined by and POSIX standard.
.It Dv CLOCK_MONOTONIC
.It Dv CLOCK_MONOTONIC_PRECISE
.It Dv CLOCK_MONOTONIC_FAST
.It Dv CLOCK_MONOTONIC_COARSE
.It Dv CLOCK_BOOTTIME
Increments in SI seconds, even while the system is suspended.
Its epoch is unspecified.
The count is not adjusted by leap seconds.
.It Dv CLOCK_UPTIME
.It Dv CLOCK_UPTIME_PRECISE
.It Dv CLOCK_UPTIME_FAST
Increments monotonically in SI seconds while the machine is running.
The count is not adjusted by leap seconds.
The epoch is unspecified.
.It Dv CLOCK_VIRTUAL
Increments only when
the CPU is running in user mode on behalf of the calling process.
.It Dv CLOCK_PROF
Increments when the CPU is running in user or kernel mode.
.It Dv CLOCK_SECOND
Returns the current second without performing a full time counter
query, using an in-kernel cached value of the current second.
.It Dv CLOCK_PROCESS_CPUTIME_ID
Returns the execution time of the calling process.
.It Dv CLOCK_THREAD_CPUTIME_ID
Returns the execution time of the calling thread.
.It Dv CLOCK_TAI
Increments in SI seconds like a wall clock.
It uses a 1970 epoch and implements the TAI timescale.
Similar to
.Dv CLOCK_REALTIME ,
but without leap seconds.
It will increase monotonically during a leap second.
Will return
.Er EINVAL
if the current offset between TAI and UTC is not known,
which may be the case early in boot before NTP or other time daemon has
synchronized.
.El
.Pp
The clock IDs
.Dv CLOCK_BOOTTIME ,
.Dv CLOCK_REALTIME ,
.Dv CLOCK_TAI ,
.Dv CLOCK_MONOTONIC ,
and
.Dv CLOCK_UPTIME
perform a full time counter query.
The clock IDs with the _FAST suffix, i.e.,
.Dv CLOCK_REALTIME_FAST ,
.Dv CLOCK_MONOTONIC_FAST ,
and
.Dv CLOCK_UPTIME_FAST ,
do not perform
a full time counter query, so their accuracy is one timer tick.
Similarly,
.Dv CLOCK_REALTIME_PRECISE ,
.Dv CLOCK_MONOTONIC_PRECISE ,
and
.Dv CLOCK_UPTIME_PRECISE
are used to get the most exact value as possible, at the expense of
execution time.
The clock IDs
.Dv CLOCK_REALTIME_COARSE
and
.Dv CLOCK_MONOTONIC_COARSE
are aliases of corresponding IDs with _FAST suffix for compatibility with other
systems.
Finally,
.Dv CLOCK_BOOTTIME
is an alias for
.Dv CLOCK_MONOTONIC
for compatibility with other systems and is unrelated to the
.Fa kern.boottime
.Xr sysctl 8 .
.Pp
The structure pointed to by
.Fa tp
is defined in
.In sys/timespec.h
as:
.Bd -literal
struct timespec {
	time_t	tv_sec;		/* seconds */
	long	tv_nsec;	/* and nanoseconds */
};
.Ed
.Pp
Only the super-user may set the time of day, using only
.Dv CLOCK_REALTIME .
If the system
.Xr securelevel 7
is greater than 1 (see
.Xr init 8 ) ,
the time may only be advanced.
This limitation is imposed to prevent a malicious super-user
from setting arbitrary time stamps on files.
The system time can still be adjusted backwards using the
.Xr adjtime 2
system call even when the system is secure.
.Pp
The resolution (granularity) of a clock is returned by the
.Fn clock_getres
system call.
This value is placed in a (non-NULL)
.Fa *tp .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The following error codes may be set in
.Va errno :
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa clock_id
or
.Fa timespec
argument
was not a valid value.
.It Bq Er EPERM
A user other than the super-user attempted to set the time.
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr adjtime 2 ,
.Xr clock_getcpuclockid 3 ,
.Xr ctime 3 ,
.Xr pthread_getcpuclockid 3
.Sh STANDARDS
The
.Fn clock_gettime ,
.Fn clock_settime ,
and
.Fn clock_getres
system calls conform to
.St -p1003.1-2008 .
The clock IDs
.Dv CLOCK_BOOTTIME ,
.Dv CLOCK_MONOTONIC_FAST ,
.Dv CLOCK_MONOTONIC_PRECISE ,
.Dv CLOCK_REALTIME_FAST ,
.Dv CLOCK_REALTIME_PRECISE ,
.Dv CLOCK_SECOND ,
.Dv CLOCK_TAI ,
.Dv CLOCK_UPTIME ,
.Dv CLOCK_UPTIME_FAST ,
and
.Dv CLOCK_UPTIME_PRECISE
are
.Fx
extensions to the POSIX interface.
.Pp
UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal emissions.
However, the
.Vt time_t
type is a simple count that does not provide a unique encoding for leap seconds,
nor a specification for what values should be used to encode a leap second.
.Pp
.Sh HISTORY
The
.Fn clock_gettime ,
.Fn clock_settime ,
and
.Fn clock_getres
system calls first appeared in
.Fx 3.0 .
